In recent years, there has been proposed a projection display apparatus (LED projector) in which light-emitting diodes (LEDs) are used as light-emitting elements for a light source.
For example, an LED projector for displaying images using a light modulation element such as a liquid crystal panel requires light-emitting elements as a light source thereof to emit light in a controlled polarization state because the liquid crystal panel is polarization-dependent. The light in the controlled polarization state refers to light in which light intensity of a polarization component in a specific direction is higher than light intensities of polarization components in other directions.
One way of controlling a polarization state is to use a polarizer. For example, Patent Literature 1 discloses a light-emitting element including a semiconductor multilayer film having a light-emitting layer and a polarization control layer (polarizer) disposed on the semiconductor multilayer film for controlling the polarization of light that is generated and emitted by the light-emitting layer.
Patent Literature 2 discloses a light-emitting element including a light emitting part for generating light in which light intensity of a polarization component in a specific direction is higher than light intensities of polarization components in other directions, the light emitting part having an active layer comprising a semiconductor in which a principal surface is a nonpolar surface or a semipolar surface. With this configuration, it is possible to control the polarization state of the emitted light without the need for a polarizer.
LED projectors are required to display images with high luminance. The efficiency of extraction of light from the light-emitting surface of a light-emitting element that is used as a light source is an effective way to realize high-luminance light sources.
Optical systems for projecting light onto a light modulation element undergo a limitation of etendue which is determined by the area of a light source and a divergence angle. In order to utilize light emitted from a light source as projection light, the value of the product of the area of the light source and the divergence angle needs to be equal to or smaller than the value of the product of the area of a display element and an acceptance angle (solid angle) that is determined by the F number of a projection lens. Therefore, to realize a high-luminance LED projector which effectively utilizes light emitted from a light source, it is necessary to increase the directivity of the light source to lower etendue.
As a method for increasing the light extraction efficiency and the directivity of light from the light source that are mentioned above, providing a photonic crystal on the light-emitting surface of a light-emitting element is known (see, for example, Patent Literature 3).
Patent Literature 4 proposes a technology of controlling the polarization of emitted light with a polarizer in addition to increasing the light extraction efficiency and the directivity of light. Specifically, a light-emitting element disclosed in Patent Literature 4 includes a reflective polarizing plate (polarizer) and an optical member (photonic crystal) that are disposed on the light-emitting side of a light emitting part for supplying light, the reflective polarizing plate transmitting a specific polarization component and reflecting a polarization component perpendicular to the specific polarization component, the optical member transmitting the light from the reflective polarizing plate and having a refractive index that periodically varies in a two-dimensional direction substantially parallel to a reference surface.